This invention relates to demand valves for breathing apparatus whereby breathable gas stored under pressure is supplied to a face-piece, hood or helmet at a rate according to the respiratory requirements of the wearer, whilst at the same time maintaining a predetermined pressure within the face-piece, hood or helmet. The predetermined pressure is set either as super-ambient, or positive, so as to prevent any inward leakage of ambient atmosphere, or may be set as a negative pressure so that gas is only supplied when the wearer inhales.
Positive pressure demand valves for breathing apparatus are well known and employ a variety of mechanisms to control the flow of gas to the wearer according to his requirements, such mechanisms being actuated by movement of a pressure responsive diaphragm exposed on one side to ambient pressure and on the other side to pressure within the face-piece such that, when the wearer inhales, causing a drop in pressure within the face-piece, the diaphragm moves inwards, actuating the valve mechanism to admit gas to the face-piece at a rate proportional to the pressure drop. When inhalation ceases, equilibrium is restored and the valve closes. In order to maintain a small positive pressure within the face-piece, the valve is biased open, typically by means of a spring bearing against the outer face of the diaphragm such that a pressure of, say, 2 millibar within the face-piece is required to move the diaphragm outwards against the spring and thus close the valve. The wearer""s exhaled breath is vented from the face-piece to the surrounding atmosphere through a simple non-return valve which is biased closed with a spring so as to only open when pressure within the face-piece exceeds ambient pressure by, say, 4 millibar. Thus it may be seen that pressure within the face-piece is continuously maintained at a level of between, say, 2 and 4 millibar above that of the surrounding atmosphere and by this means, any leakage due to damage or imperfect sealing of the face-piece, can only be outwards, so preventing any ingress of ambient atmosphere to the face-piece.
For many applications, it is required to provide the demand valve with a manually operated bypass valve in order to supply a controlled continuous flow of breathable gas to the wearer independently of the normal demand operation.
An objective of the present invention is to provide a demand valve of simplified construction and assembly, wherein accurate positioning of moving parts is achieved with the minimum number of components.
A further objective of the present invention is to provide a two-stage demand valve whose performance is reliable and predictable, but which requires a minimum of precision-made components
A further objective of the present invention is to provide a manual bypass arrangement for use in demand valves, incorporating a simplified construction with a minimum number of components, which provides to a wearer of the demand valve the possibility of a continuous flow of breathable gas to the facepiece.
One aspect of the present invention provides a pilot operated demand valve which is efficient, reliable and predictable in operation and is of small physical size and simple construction. A second aspect of the invention provides a bypass arrangement for a demand valve.